It is difficult to meet the density and throughput demand on the line cards operating with WDM optics, also called colored optics or line optics. If WDM optics cannot meet such demands and consequently are perceived as a waste of router switching capacity, then grey optics—the alternative to WDM optics—may be connected to third party external transponder shelves.
The gap between WDM optics and grey optics results from the inclusion in WDM optics of at least one temperature controlled tunable laser source. Such laser sources occupy a large part of WDM optics such as WDM optical modules. For example, over 30% of the real estate could be occupied by laser sources, such that the laser sources set a floor on the size of WDM optical modules and a ceiling on the density of WDM optical modules.
The laser source is often the only element of WDM optical modules that requires a thermo-electric cooler (TEC). Thus compared to other elements on the WDM optical module, the laser source imposes the strictest requirements on the case temperature and heat dissipation.
The laser source height is difficult to reduce. For example, the integrated laser assembly height adds with the printed circuit board assembly (PCBA) and prevents the assembly from fitting into the 9.5 mm dimension of CFP8/CFP4 modules.
The laser source can consume, for example, more than 30% of the power of WDM optical modules, including the TEC. Typical numbers are about 3 W. Optimistic projections for the future are around 2 W.
The optical power of micro-integrable tunable laser assemblies (μITLA) in the modules can generate insufficient optical power with acceptable power consumption, or else generate sufficient optical power with unacceptable power consumption. An optical amplifier may be required in order to overcome insufficient optical power.
The laser source is not integrated in the transmitter/receiver. The laser source is a separate chip coupled with a fiber to the transmitter optical sub-assembly and receiver optical sub-assembly (TOSA and ROSA). Prospective gains from integration are limited in terms of size and power.
When WDM optics are a coherent transceiver, WDM optics also include a very powerful DSP and power consuming optoelectronics (e.g., a polarization multiplexed in-phase and quadrature (PM-IQ) modulator with a quad-driver and 4 digital-to-analog converters (DACs), and an integrated coherent receiver (ICR) paired with 4 receivers and 4 transimpedance amplifiers (TIAs)).
Due to the inclusion of such elements, it is difficult to reduce the size and power consumption of WDM optics modules down to grey optics modules. Low-power digital signal processing (DSP) and low-power optics, integration on-board, and so on are all helpful aspects that do not give a fundamental advantage to WDM optics; when grey optics uses the same such technology, grey optics consume less power and occupy less space than its WDM counterpart.